1. Field of the Invention
The invention concerns the treatment of waste water or drinking water to eliminate therefrom mineral or organic particulate and colloidal materials in suspension and nitrogen-containing and phosphorus-containing pollutants.
2. Description of the Prior Art
There are two normally contradictory water treatment techniques: physical-chemical treatment and biological treatment.
Physical-chemical treatment uses chemical reagents adapted to cause coagulation of colloidal impurity particles by binding them to form floc from which bulky and heavy clumps are then obtained. The coagulant reagents are hydrolyzed and precipitate in the form of discrete particles whose structure depends on the physics/chemistry of the reagents and which have a very high specific surface area, often in the order of 1 000 m.sup.2 /g. Turbulence induced in the water causes the particles to interact with the organic materials and the materials in suspension to neutralize their surface charge and absorb them to form a microfloc, constituting seed particles from which a future floc (i.e. a future visible clump) is obtained by growth of the microfloc. The water laden with floc is then transferred to a separator reactor from which a clarified effluent and sludge made up of this floc are recovered separately. A particularly high-performance example of such physical/chemical treatment is described in French Patent No. 2,631,021 which describes a method of purifying water with a prolonged coagulation phase enabling, with moderate concentrations of reagents and appropriate agitation, fast elimination of materials in suspension, including colloidal materials. The floc can be fixed or free, depending on whether it is grown on granular supports or in the free state. Separation of the effluent from the sludge can follow on immediately from coagulation/flocculation or be separated from the latter by intermediate stages in which a greater or lesser degree of turbulence is maintained.
The coagulation/flocculation reagents can be of highly varied types including iron salts (such as ferric chloride), aluminum salts (especially chlorides and sulfates) and salts of other metals; they can equally well be polymers.
There are various known methods for determining the quantity and the nature of the reagents to be added to a given water in order to coagulate/flocculate the organic pollution thereof (C, usually defined by the Biological Oxygen Demand BOD, by the Chemical Oxygen Demand COD or even by the Total Organic Carbon TOC) or particulate pollution (Materials In Suspension: MIS). One of these methods is the JAR-TEST. The previously mentioned French Patent No. 2,631,021 describes how to adapt the conditions of the coagulation/flocculation process (with particular reference to agitation) to obtain the required flocculation using the minimum quantities of reagents.
Biological treatments use bacteria (sometimes referred to collectively as "biomass") which can be free bacteria (in "activated sludge" systems) or fixed bacteria (in reactors containing granular supports on which the bacteria are grown and which constitute filters). These bacteria obtain their nourishment from the water to be treated. In practice steps are taken to promote the growth of a particular bacteria population, chosen for its ability to break down a given type of pollution.
Biological treatments are used in particular to eliminate nitrogen-containing pollution from the water to be treated. This pollution is eliminated in two stages, using two different families of bacteria.
Liquid ammonia (in practice the NH.sub.4.sup.+ ion) is eliminated by autotrophic nitrifying bacteria. These bacteria must be kept in an aerobic medium, which means that aeration is required. They oxidize the NH.sub.4.sup.+ molecule using the injected oxygen to form nitrites (NO.sub.2, Nitrosomonas bacteria) and then nitrates (NO.sub.3, Nitrobacter bacteria).
The nitrates are eliminated by heterotrophic denitrifying bacteria which, using a carbon-containing substrate and in the absence of free oxygen (the medium must be anoxic), take oxygen from the nitrates, releasing nitrogen in gaseous form.
Maintaining a given bacteria population obviously entails maintaining a certain number of operating parameters (aeration or no aeration, provision of suitable substrates in a form that can be assimilated) and avoiding any phenomena which could have toxic effects on the bacteria, with the risk of a reduction in biological yield or even poisoning of the bacteria. This is why water treatment agencies are disinclined to envisage the injection of reagents in substantial quantities at the entry to a biological reactor.
A known and particularly effective process for the biological purification of nitrogen-containing pollution consists in causing the water to be treated to flow upwards through a denitrification reactor containing denitrifying bacteria in an anoxic medium and then through a nitrification reactor containing nitrifying bacteria in an aerobic medium, a substantial fraction of the effluent leaving the second reactor being recycled by mixing with the effluent to be treated before it enters the first reactor. The denitrifying bacteria therefore degrade the nitrates from the second reactor using the carbon-containing substrate constituted by the carbon-containing pollution in the effluent to be treated. The biological factor limiting the elimination of nitrates in this case is in practice the quantity of carbon present in the effluent to be treated in a form that can be assimilated by the bacteria.
A process of this kind is described in French Patent No. 2,673,618. This patent describes a process for nitrifying and denitrifying polluted water by biological treatment in an aerobic medium and then an anaerobic (or anoxic) medium in which the untreated water, without preliminary settling, is caused to flow upwards through an anoxic first reactor with free biomass in the form of a highly concentrated sludge bed with a high upward flowrate. The effluent from the first reactor is transferred to an aerobic second reactor. Some of the treated water, overflowing from the second reactor, is recycled to the bottom of the first reactor. The total flowrate at the entry to the first reactor (water to be treated plus recycled water) preferably represents an upflow velocity of at least 3 m/h. The flow entering the bottom of the first reactor produces a granular sludge made heavier by the material in suspension in the untreated influent. The concentration of this sludge in the first reactor is preferably between 30 g/l and 100 g/l. Agitation at the bottom of the first reactor improves distribution and enables bubbles of nitrogen to escape. The upper part of the first reactor is advantageously equipped with a lamellar (parallel plate) settling tank. The untreated water has usually been screened and had sand oil removed from it beforehand (but without any primary settling being necessary) and can have reagents added to it adapted to increase the settling rate of the sludge, for example flocculating agents such as alum, ferric chloride or polymers. Lime can also be added to form carbonates and precipitate the phosphorus.
Another denitrification-nitrification process is described in European Patent No. 0,522,966 (TAMBO) which proposes a flocculation stage to form floc laden with phosphorus, a stage of mixing with a recycled fraction of nitrified water, a denitrification stage, a solid separation stage and a nitrification stage at the outlet from which the recycled fraction is taken. Polymer flocculating agents are used, for example, at a concentration of about 0.1 ppm-20 ppm (equivalent to 0.1 mg/l-20 mg/l). Further polymer flocculating agents can be added prior to the denitrification stage.
The idea of a denitrification reactor with no granular material had already been described by A. KLAPWJIC, in particular in "The Application of an upflow reactor in the denitrification step of biological sewage treatment", KLAPWJIC, JOL and DONKER, published in Water Research Vol 13, pp 1009-1015, Pergammon Press Limited 1979. Tests have been carried out with a small reactor (14.7 l) and sludge containing denitrifying bacteria agitated intermittently (10 minute intervals between agitation for 3 seconds at 120 rpm). The water to be treated had been allowed to settle beforehand, the upflow velocity was 0.12 m/h and the mass concentration of the sludge was 30 g/l.
An object of the invention is to eliminate organic and nitrogen-containing (even phosphorus-containing) particulate and colloidal (materials in suspension) pollution, offering purification performance levels exceeding the prior art levels. In particular, it is directed to purification of urban or industrial waste water (and drinking water) in conformance with the E standards (MIS&lt;30 mg/l, COD&lt;90 mg/l, nitrogen-containing pollution in the form NH.sub.4.sup.+ and NO.sub.3.sup.- &lt;20 mg/l), or even the F standards (MIS&lt;15 mg/l, COD&lt;M 50 mg/l, nitrogen-containing pollution in NH.sub.4.sup.+ and NO.sub.3.sup.- form&lt;10 mg/l), and preferably the PT1 standards (phosphorus&lt;2 mg/l) or even the PT2 standards (phosphorus&lt;1 mg/l), using a process that is fast, efficient and economical and which does not require a costly and large-size installation.
To this end the invention teaches combining a specific physical/chemical treatment (thorough coagulation/flocculation) with a specific biological treatment (denitrification with dilution in the nitrified water obtained in a second stage).
It is important to emphasize that water treatment agencies have been reluctant to accept any such combination, for the following reasons:
the short-term and long term effects of physical/chemical reagents on bacteria and on their biological yield is unknown or at best only poorly understood; all the more so in that there may be negative synergistic effects (with attendant risks of toxicity) between reagents and/or breakdown products which do not in isolation have deleterious effects,
it is logical to think that the carbon which can be assimilated by the bacteria and which is therefore needed for the denitrification process (in theory this is only part of the BOD which is itself only part of the COD), is mainly in soluble form. It is clear that flocculation tends to reduce the quantity of soluble carbon available in the water, due to adsorption, and therefore tends to prevent the carbon that can be assimilated reaching the bacteria freely. It is therefore logical to fear that physical/chemical coagulation/flocculation treatment could reduce the quantity of carbon available in the water in a form that can be assimilated by the bacteria and therefore at least inhibit to a greater or lesser degree the biological denitrification process. It has therefore seemed unthinkable to envisage physical/chemical treatment pushed to the extent of virtually total elimination of the carbon-containing pollution that could be assimilated immediately before biological treatment employing heterotrophic bacteria using the carbon-containing substrate to break down the nitrates.